It is common to employ cement to repair bones in a variety of clinical scenarios.
For example, compression fractures of the vertebrae, which are a common occurrence in older persons, cause pain and/or a shortening (or other distortion) of stature. In a procedure known as vertebroplasty cement is injected into a fractured vertebra. Vertebroplasty stabilizes the fracture and reduces pain, although it does not restore the vertebra and person to their original height. In vertebroplasty the cement is typically injected in a liquid phase so that resistance to injection is not too great. Liquid cement may unintentionally be injected outside of the vertebra and/or may migrate out through cracks in the vertebra.
In another procedure, known as kyphoplasty, the fracture is reduced by expanding a device, such as a balloon inside the vertebra and then injecting a fixing material and/or an implant. Kyphoplasty reduces the problem of cement leakage by permitting a lower pressure to be used for injection of the cement.
In general, polymeric cements become more viscous as the polymer chain grows by reacting directly with the double bond of a monomer. Polymerization begins by the “addition mechanism” in which a monomer becomes unstable by reacting with an initiator, a volatile molecule that is most commonly a radical (molecules that contain a single unpaired electron). Radicals bond with monomers, forming monomer radicals that can attack the double bond of the next monomer to propagate the polymer chain. Because radicals are so transient, initiators are often added in the form of an un-reactive peroxide form which is stable in solution. Radicals are formed when heat or light cleaves the peroxide molecule. For applications in which high temperatures are not practical (such as the use of bone cement in vivo), peroxide is typically cleaved by adding a chemical activator such as N,N-dimethyl-p-toluidine. (Nussbaum D A et al: “The Chemistry of Acrylic Bone Cement and Implication for Clinical Use in Image-guided Therapy”, J Vasc Interv Radiol (2004); 15:121-126; the content of which is fully incorporated herein by reference).
Examples of commercially available viscous bone cements include, but are not limited to, CMW® Nos. 1, 2 and 3 (DePuy Orthopaedics Inc.; Warsaw, Ind., USA) and Simplex™-P and -RO (Stryker Orthopaedics; Mahwah, N.J., USA). These cements are characterized by a liquid phase after mixing and prior to achieving a viscosity of 500 Pascal-second. In a typical use scenario, these previously available cements are poured, while in a liquid phase, into a delivery device.
There have also been attempts to reduce cement leakage by injecting more viscous cement, for example, during the doughing time and the beginning of polymerization. However, the viscous materials, such as hardening PMMA, typically harden very quickly once they reach a high viscosity. This has generally prevented injection of viscous materials in orthopedic procedures.
Some bone fixing materials, such as polymethylmethacrylate (PMMA), emit heat and possibly toxic materials while setting.
U.S. patents and publication U.S. Pat. Nos. 4,969,888, 5,108,404, 6,383,188, Nos. 2003/0109883, 2002/0068974, U.S. Pat. Nos. 6,348,055, 6,383,190, 4,494,535, 4,653,489 and 4,653,487, the disclosures of which are incorporated herein by reference describe various tools and methods for treating bone.
US patent publication 2004/0260303, the disclosure of which is incorporated herein by reference, teaches an apparatus for delivering bone cement into a vertebra.
Pascual, B., et al., “New Aspects of the Effect of Size and Size Distribution on the Setting Parameters and Mechanical Properties of Acrylic Bone Cements,” Biomaterials, 17(5): 509-516 (1996) considers the effect of PMMA bead size on setting parameters of cement. This article is fully incorporated herein by reference.
Hernandez, et al., (2005) “Influence of Powder Particle Size Distribution on Complex Viscosity and Other Properties of Acrylic Bone Cement for Vertebroplasty and Kyphoplasty” Wiley International Science D01:10:1002/jbm.b.30409 (pages 98-103) considers the effect of PMMA bead size distribution on setting parameters of cement. Hernandez suggests that it is advantageous to formulate cement with a liquid phase to facilitate injection. This article is fully incorporated herein by reference.
U.S. Pat. No. 5,276,070 to Arroyo discloses use of acrylic polymers with a molecular weight in the range of 0.5 to 1.5 million Daltons in formulation of bone cement. The disclosure of this patent is fully incorporated herein by reference.
U.S. Pat. No. 5,336,699 to Cooke discloses use of acrylic polymers with a molecular weight of about one hundred thousand Daltons in formulation of bone cement. The disclosure of this patent is fully incorporated herein by reference.